Method and apparatus for arresting a missile



March 1967 J. s. STRANGE ETAL 3,309,044

METHOD AND APPARATUS FOR ARRESTING A MISSILE Filed July 27, 1965 4Sheets-Sheet 1 STROKE FT.

$-22? m ITILIFII'FILILIL INVENTORS JOHN s. STRANGE BY ARTHUR G.CONDOD|NAFLOYD SILVER 7M 41 M, 1%? J. 5. STRANGE ETAL 3,309,044

METHOD AND APPARATUS FOR ARRESTING A MISSILE Filed July 27, 1965 4Sheets-Sheet 2 I INVENTORS JOHN S.STRANCE BY ARTHUR G.CONDODINA FLOYDSILVER THEM/ 3! W March 14, 1967 J. 5. STRANGE ETAL METHOD AND APPARATUSFOR ARRESTING A MISSILE 4 Sheets-Sheet 5 Filed July 2'7, 1965 HAN W AW!Fig 70 INVENTORS JOHN SSTRANCE BY ARTHUR G.CONDODINA FLOYD SlLVER March14, 1967 J. 5. STRANGE ETAL 3,309,044

METHOD AND APPARATUS FOR ARRESTING A MISSILE Filed July 2'7, 1965 4Sheets-Sheet 4 V v Q, (L P 3 ;L\Q I l 17* M 1 v I N VEN TORS JOHN$.5TRANCE y ARTHUR G.CONDOD|NA United States Patent Ofilice 33%,044Patented Mar. 14, 1967 3,309,044 METHOD AND APPARATUS FOR ARRESTING AMISSILE John S. Strance, Drexel Hill, Arthur G. Condodiua, Philadelphia,and Floyd Silver, Secane, Pa, assignors to E. W. Bliss Company, Canton,()liio, a corporation of Delaware Filed .Iuly 27, 1965, Ser. No. 475,1647 Claims. (Cl. 244-4110) This invention relates to a missile arrestingsystem and method for absorbing the kinetic energy of a missile while inflight so as to bring it to a controlled stop within a limited verticaldistance.

More specifically, the invention will be described with reference to thearrestment of a missile in free flight by means of a rotary, reel-type,arresting engine similar to that disclosed in US. patent applicationSer. No. 441,560 filed Mar. 22, 1965, which employs hydraulicallyactuated friction disc reel brakes as the energy absorbing medium,however it will be appreciated that any suitable energy absorbing devicemay be employed so long as it has a quick response performancecapability.

In test firing missiles it is customary to obtain tele- Inetry databeginning with the initial lift-01f and continuing for a specifiedperiod depending on the type of flight information to be obtained. Wherethe telemetric data desired is primarily available only in the firstseveral feet after firing, the remainder of the flight is merely anexercise in ballistics with the missile impacting at a specified pointdown range. In the absence of complicated soft landing recoveryequipment, this results in the destruction of the missile shell and anytelemet-ric equipment on board. Obviously therefore, such one-shottesting has been a substantial factor in the soaring costs of missiletesting programs.

Another problem is that advanced clearance is required from governmentauthorities when the trajectory of a missile will take it above acertain altitude. Merely scheduling the test thus becomes a difficultyin itself.

These and other problems are overcome with the present invention whichhas as a primary purpose the provision of a missile arresting method andapparatus capable of engaging the missile after a period of sustainedfree flight and then bringing it to a controlled stop in a restrictedspace.

In accordance with the invention, a missile is fired upon a course at agiven speed and after a period of free flight is restrained by anarresting gear including a rotatable reel and a linear purchase tapeconsiderably wider than it is thick so as to permit coiling upon itselfabout the reel in ever increasing convolutions. The running end of thetape is engaged by the missile at the end of its free flight period andcarried aloft by it. The tape is woven of synthetic yarns having asufiiciently low modulus of elasticity so that the portion paid-outbetween the reel and missile will stretch longitudinally and maintainengagement with the missile until the reel can accelerate to supply thetape needed to follow the missile. Rotary friction brakes on the reelreduce the rate of tape payout thus slowing the missile to a stallingspeed. And a speed sensing control is arranged to program the brakepressure so that energy stored in the tape as stretch is relieved priorto bringing the missile to a controlled stop at its apogee.

Further in accordance with the invention, the missile is guidedvertically and a plurality of reel-type arresting engines are positionedequidistant from the missile launcher, the arrangement being such thatthe missile is launched with each tape attached to its shell by means ofa loose bridle, the length of which equals the desired free flightdistance and the running end of each purchase tape is releasablyanchored adjacent the launcher so that after launch each tape and bridlemake an oblique angle with the vertical when tensioned at the moment ofmissile impact on the system.

Further in accordance with the invention, each arresting engine includesa stator member, a reel rotatably mounted on the stator member, aplurality of friction disc elements carried by the reel and extendingradially in interleaved fashion with stationary friction disc elementscarried by the stator member and hydraulic control means for applyingbraking pressure from opposite sides of the reel in accordance with amissile braking program, the reel, rotary friction disc elements, andtape stack on the reel constituting a low inertia system capable ofquick response to the longitudinal stress waves generated at the runningend of the tape.

The invention also contemplates a method for arresting a missilecomprising the steps of restraining the missile after a period of freeflight with a woven purchase tape made of synthetic yarn placed intension by means of oblique impact with the missile, paying outadditional tape from a reel upon which the tape is coiled at a ratesuflicient to prevent excessive tape strain, reducing the rate of tapepayout after the reel has accelerated to missile velocity and therebyslowing the missile to a stalling speed, increasing the rate of tapepayout as the missile approaches zero velocity to relieve tape strain,and halting tape payout after the strain has been relieved and as themissile comes to rest.

The principal object of the invention is to provide an arresting gearand method for arresting a missile tethered by means of a purchasemember to an energy absorbing means in such a fashion that after aninitial period of free flight the missile impacts with the arrestinggear and is brought to a controlled stop by the energy absorbing meanswithin a limited distance.

Another object is to provide an arresting gear employing a woven tape ofsynthetic fibers as the purchase member characterized by having a lowmodulus of elasticity so that the tape is capable of suflicientelongation at oblique impact angles with the missile whereby its ulti-'mate stress is not exceeded before the tape payout means can be set inmotion to feed additional tape into the system.

Another object is to provide an energy absorbing and purchase tapepayout gear having a relatively low effective mass with its complementof tape stored thereon so as to readily respond to the longitudinalstress wave impulses transmitted from the running end of the tape.

These and other objects will become apparent by reference to thefollowing description and drawings wherein:

FIGURE 1 is a front elevation schematic illustration of a missilelaunching installation utilizing diametrically opposed reel-typearresting engines as contemplated by the invention;

FIGURE 2 is a side view of the installation shown in FIGURE 1illustrating the missile position at the apogee of its flight path;

FIGURE 3 is a diagram showing a family of curves representing therelationship of oblique impact stress on a nylon tape system;

FIGURE 4 is a diagram showing the relationship between tape tension,missile velocity and payout rate at various missile heights during thedynamic period of the arrestment;

FIGURE 5 is a diagram depicting arresting performance during the entirearrestrnent stroke;

FIGURE 6 is a perspective view of an arresting engine of the type usedin the missile arresting system shown in FIGURE 1;

FIGURE 7 is a partial cross-sectional view of the arresting engine inFIGURE 6 taken approximately along line 7-7 of FIGURE 6;

FIGURES 8 and 8a are enlarged views showing the arrangement of the tapeto tape connector and tape to missile connector for the missilearresting system of FIGURE 1; p

FIGURE 9 is a diagram of the speed. responsive hydraulic control systemfor operating the rotary friction brakes of the arresting engine inFIGURE 7;

FIGURES l and 10a are enlargements of the braking program control camand valve of the hydraulic system shown in FIGURE 9 and a cross-sectionof the pressure control valve element showing the pressure relief portstherein; I

FIGURES l1 and 11a are schematic views representing a modification ofthe invention as a below ground missile launching installation in whichthe missile engages parallel horizontal pendants at ground level; and

FIGURE 12 is another modification of the invention showing the use of asingle arresting engine with vertical payout of the purchase tape.

Referring now to the figures where the showings are for the purpose ofillustrating a preferred embodiment of the invention only and not forthe purpose of limiting same, FIGURES 1 and 2 show in more or lessschematic form a missile launching and testing installation 10 includinga launching tube 12 housing a missile 13 in a position ready for firingalong a vertical flight path 15. The missile 13 reaches the apogee ofits flight path 15 at the completion of the test as shown in FIGURE 2.Cable guides 16, 17 are suspended at their upper ends from a boom 18 bymeans of a winch and cable mechanism generally indicated at 19 and areanchored at their lower ends within the launching tube 12 in any knownmanner. The missile 13 carries diametrically opposed, one-way cable gripdevices 22, 23 which ride on the cables 16,17 respectively as themissile is propelled aloft. The cable grippers 22, 23 may be of anyknown type which permit the cable to slip in one direction yet lock and.hold against movement in the opposite direction.

As will be appreciated by those familiar with missile testingprocedures, the installation shown in FIGURES 1 and 2 is primarilyuseful in the situation where the telemetric data desired is availableduring the first instant after liftoff which means the remainder of themissile flight is normally useless so far as taking readings on missileflight behavior are concerned. In the example chosen for illustratingthe invention, the test is compieted after th missile has been propelledaloft a free flight distance of sixty-seven feet as indicated by theelevated missile position in FIGURE 1. This corresponds to the end ofthe test and the beginning of the arrestment period designed to bringthe missile 13 to a controlled stop at its apogee (FIGURE 2). Throughoutthe test the missile is tethered to a missile arresting gear 24 which isdesigned to not interfere with the period of free flight as will beexplained hereinafter.

The arresting gear 24 generally includes two arresting engines 25, aseach spaced at equal distances from the launching tube 12 and. 180degrees apart facing the missile. Each arresting engine 25, 25 includesa rotatable reel 27, 28 adapted to store a continuous coil of purchasetape 29, 30 to be paid out over sheaves 31, 32 during the arrestmentstroke. Prior to launch. each tape 29, 30 is releasably anchored at theperiphery of the launching tube 12 in a manner to be described and isconnected to the missile 13 by the means of a loose bridle 34, 35constructed in the same manner as the tapes 29, 30. The length of thebridles 34, 35 is suflicient to allow for the period of free flightbefore the arresting gear is impacted. At the battery position or momentof impact, the anchored ends of purchase tapes 29, 30 are simultaneouslyjerked. free as the bridles 34, 35 are tensioned as shown by the dottedlines in FIGURE 1. The purchase tapes 29, 30 are then paid out from thereels 25, 26 at a programmed controlled braking rate so that the missile1'3 is brought to a stop (FIGURE 2) whereupon it is held by the boom 18from falling back to the ground. The boom Winch mechanism 19 may then beoperated to lower the missile and the guide cables 16, 17 thus saving itand the telemetric equipment on board for another test.

To better understand the present invention, a more detailedconsideration of the theoretical concepts involved should be considered.By way of analogy, the arrestment of an aircraft on short runwaysprovides a background for the arrestment of a missile. For a thoroughdeveio'pment of the principles involved in arresting an aircraft,reference is made to United States Reissue Patent No. 25,406 issued June25, 1963, which discusses in considerable detail the arrestingproperties of arresting gear employing a woven tape of synthetic yarnsuch as nylon as a purchase member. Briefly, these benefits ariseprimarily from the fact that such synthetic yarns have a low modulus ofelasticity as compared to that of steel. It will be readily recognizedthat a steel cable stretched across a runway must immediately acceieratefrom Zero to the speed of the aircraft or fail in tension. There is afinite time before the stress Wave front traveling at the speed of soundcan reach and actuate the payout mechanism and provide the necessaryextra cable at the center span demanded by the movement of the aircraftdown the runway thus the cable must either be susceptible to continuousstretching (strain) at some rate in feet per second until the systemphysically moves to provide the fed in, or the arresting gear simplymust fail in tension due to separation of the purchase member. Otherwisestated, there is an upper limit to longitudinal stress which can beapplied. to a steel cable in excess of which the cable will fail intension. This upper limit condition may be expressed by the equation:

where S equals the stress in p.s.i.; E is the modulus of elasticity ofthe cable in p.s.i.; V is the velocity of impact on the cable in feetper second; and C is the speed of sound in the cable in feet per second.An accepted modulus of elasticity for steel is about 12 l0 p.s.i., a Cvalue of 10,000 feet per second and a maximum stress value of 240,000p.s.i. Substituting these values in the equation it will be seen thatthe maximum velocity of impact sustainable by the cable cannot exceed200 feet per second. As disclosed in the above-mentioned patent, thestretch necessary in the pendant, while not available from a steel cablealone, can be provided by the use of a nylon tape connected at theopposite ends of the steel pendant. The nylon tape providessubstantially all the stretch needed while at the same time transmittingthe initial longitudinal stress wave to the payout means putting it intomotion before the maximum stress capacity of the nylon tape can beexceeded. Referring once again to the above equation, it may be seenthat with nylon having a maximum strength capacity of about 50,000 psi,21 modulus of elasticity of around 300,000 psi. and a speed of sound of5,000 feet per second; the maximum allowable impact velocity isincreased. from 200 feet per second to approximately 833 feet persecond. By comparison, there is approximately a fourfold advantage inthe use of a nylon tape primarily because of the much lower modulus ofelasticity and While the maximum stress of 240,000 p.s.i. for steel andthe speed of sound at 10,000 feet per second are Ofisetting factors infavor of the use of steel, they do not compensate for the fact that themodulus of elasticity of steel is about 40 times greater than that ofnylon.

T-hees same principles as applied to the arrestment of an aircraft alsohold true for the arrestment of a missile, however, an additionalconsideration is required in the case of a missile due to the geometryof the system at the instant of impact or at the battery position. Thatis, in the normal case of aircraft arrestment, the pendant is stretchedtaut across the runway so as to extend perpendicular to the glide pathof the aircraft in the battery position. While in the case of arrestmentof a missile, the missile initially engages the purchase member whichextends at an oblique angle to the flight path known as the angle beta.

The angle beta at which the purchase tape 29, 30 and bridle extensions34, 35 first become taut after being carried aloft by the missile isdetermined by the spacing of sheaves 31, 32 and the requirements of thetest as to the vertical free flight distance. In the example selected toillustrate the invention, the vertical free flight distance is 67 feetfrom launch and the sheave to sheave distance is 200 feet measuring onthe horizontal between the sheaves 31, 32. Thus the angle beta formedwith the vertical by the taut purchase members 29, 30, 34, 35 at theinstant of impact is approximately 5 6 degrees (dotted line position inFIGURE 1) and the straight line distance from each sheave 31, 32 to themissile is approximately 120 feet. This is the beginning of thearrestment stroke and is analogous to the battery position of a crossdecked pendant in an aircraft arresting installation only in that casebeta in 90 degrees. The main difference comes from the obliquity of theengagement. The eifect of oblique engagement may be more fullyunderstood by referring to FIGURE 3 which shows a family of curves forangles of beta between zero and 90 degrees for oblique impact stress ona nylon tape. The two extremes are direct longitudinal engagement atwhich beta is zero degrees and perpendicular engagement where beta is 90degrees. The curves show the relationship of the ratio of stress tomodulus of elasticity plotted on the ordinate versus the ratio of theengagement velocity to speed of sound in nylon plotted on the abscissa.These values may be obtained by referring to the above formula EV S- CFor the purposes of illustration it will be assumed that the missilevelocity at battery position is in the order of 270 feet per second,thus a value of V/ C is obtained of 5.4 and by following the beta curveof 56 degrees, we find that the value of S/E is 3.5 and since for nylonE equals 300,000 p.s.i.; the impact stress is 10,500 p.s.i. For a tape 7inches wide and 0.19 inch thick this is a tension of 14,000 pounds. Bycomparison, with an angle of beta at 90 degrees and assuming the sameengaging velocity, we find that the value for S/E is approximately 1.7.Again multiplying by the modulus of elasticity of 300,000 p.s.i. thisgives an impact stress of about 5,100 p.s.i. or approximately half thestress that was obtained on the oblique engagement. Thus the strain onthe purchase tape varies inversely with the size of the angle beta,i.e., for lower angles of beta higher longitudinal stress waves will begenerated in the purchase tape. Thus the requirements for tape stretchat oblique engagement will greatly exceed the requirements where theengagement is head on.

The dynamic period of missile arrestment is the period of time it takesthe reels 27, 28 to accelerate to a payout speed equaling the speed ofthe missile. The performance of the arresting gear 24 during the dynamicperiod may best be understood by reference to the chart shown in FIGURE4 which breaks down the dynamic period into four increments beginningwith the initial impact or battery position. Actually the incrementsselected are an arbitrary choice to facilitate discussion and the actualcurve plots would be smoothed out versions of those shown in FIGURE 4.With the ordinate of the chart representing velocity in feet per secondand tape tension in thousands of pounds and the abscissa missile heightabove ground in feet and using the previous example of a free flightdistance of 67 feet, it is seen from the middle curve that at thebattery position, tape tension goes from approximately zero to 14,000pounds with a missile velocity of 270 feet per second (upper curve).During the time it takes for the longitudinal stress wave due to initialimpact to travel down the purchase members 29, 30, 34, 35, and back tothe arresting engines 25, 26, the missile will have traveledapproximately 17 feet vertically which means a change in length of thepurchase members of approximately 12 feet, all of which must come fromstretch within the tape itself. The second increment of the chart isselected to begin at the instant the longitudinal stress wave reachesthe arresting engines 25, 26 which represents a time lag of about 0.064second. The reels 27, 28 now begin to accelerate under the influence ofthe 14,000 pound initial impact force and assuming a 40 percent tensionincrease at the reel due to reflection of the stress wave, reelacceelration is determined by the formula:

where A is acceleration in feet per second squared; T is the tapetension in pounds and Me is the effective mass of the rotatable reel andits complement of stored tape giving an acceleration of 1565 feet persecond squared. Assuming an increment of time of 0.05 second, the reelswill be playing tape out at the end of the second increment at about 78feet per second for an average velocity of about 40 feet per second(bottom curve). This then is a payout length of about 2 feet. However,during this time increment, the missile will have traveled another 13feet for a total of about 97 feet above ground bringing the total tapestretch requirements, by virtue of geometry of the system, to 20 feet.Since now the payout reels are rotating and have dumped approximatelytwo feet of additional tape in the system, the net requirement forstretch in the tape itself is 18 feet or an additional stretch over thatimposed by initial impact of approximately 6 feet. The tape stress isnow up to about 15,100 p.s.i. (20,000 pounds tension) or approximatelyhalf of the allowable stress for nylon of 30,000 p.s.i. Thus with themissile having traveled 30 feet from the battery position, the tapestress has increased to approximately half its maximum allowable stressand the velocity of the missile has decreased from 270 feet per secondto about 260 feet per second.

In contrast, it will be readily apparent that under the same conditionsa purchase member having a high modulus of elasticity such as steelwould fail before the reels 27, 28 could be accelerated sufficiently tosupply the needed feed in.

For the example selected, at the end of the dynamic period the missilevelocity will be slowed to 250 feet per second at a stroke height ofabout 123 feet. By this time the payout reels 27, 23 will haveaccelerated to the point where they are dumping tape into the system atthe same velocity as the missile or approximately 250 feet per sec- 0ndand the tape tension will level off at 24,000 pounds. At this point andto prevent overspeed of the reels, the reel brakes are engaged in aprogrammed fashion to bring the missile to a controlled stop (FIGURE 2)within about 200 feet. The total time from launch to stop at the apogeeis less than 1 second.

FIGURE 5 is a curve which represents the overall performance of thearresting gear showing the retarding force in pounds imposed on themissile at the various heights. It should be noted that after thedynamic period which ends at about 123 feet, a relatively flat curve isshown dropping off rapidly near the end of the arrestment stroke. Thesignificance of this portion of the curve will become more apparent fromthe following discussion.

- Referring now to FIGURES 6 and 7, the arresting engines '25, 26 arereel-type units similar to those disclosed in the aforementioned UnitedStates patent application Ser. No. 441,560. Since the engines 25, 26 areidentical, a description with respect to one will be understood asapplying equallyto the other. With respect to engine 25, the purchasetape 29 is a woven construction of synthetic yarns considerably widerthan it is thick so as to permit coiling upon the reel 27 of the engine25. For details of such a Woven tape reference is made to US.

patent application Ser. No. 461,123 filed lune 3, 1965. The tape 29 isstored on the reel 27 and is paid out through a tape duct 37 and thesheave 31. After arrestment the tape can be detached from the missileand the etraction motor 38 clutched in to recycle the reel 27 and rewindit to a position ready for the next ar'restment. The tape 29 isconnected to the bridle extension 34 by means of a tape to tapeconnector 41) shown in FIGURE 8 which is merely a lightweight metalblock having transverse slots 42 in each end adapted to receive the endsof tape 29 and bridle extension 34, each of which is molded to theconfiguration of the slots 42 so as to slide laterally into the slots 42thus holding the ends securely against longitudinal movement. Since thetapes are of a woven nylon material it is preferable to form thecontoured ends by means of molding a hard, plastic material around themas disclosed in US. patent application Ser. No. 438,459 filed Feb.19,1965 and now Patent No. 3,263,289. The tape to tape connector 40 hasa rod 45 extending laterally from opposite sides thereof which rests inears 4-7 projecting outwardly from the lip of the launching tube 12. Allof the slack is taken out of the tape 29 by a slight pretensioningagainst the cars 47. The bridle extension 34 is not under any tensionand passes over the lip of the launching tube 12 for attachment with themissile 13 by tape to missile connector 423. The connector 48 ispivotally mounted at 49 to a pair of laterally projecting lugs 51)mounted on a circular band 51 girding the missile. The swivelarrangement permits the connector 43 to swing as the missile islaunched.

It is important to note in FIGURE 7 that the reel 27 is rotatablymounted at 53 upon a stationary drum 54 to provide a low inertiaconstruction. The reel 27 is radially spaced from the drum 54 to definea brake chamber 56 sealed at opposite ends by rotary seals 57. A rotaryfriction brake 58 is mounted within the brake chamber 56 and comprisesan annular array of hydraulic brake actuators 60 mounted on oppositeends of the drum 54 and arranged in diametrically opposed relationship.A plurality of brake stator members 61 are guided in keyways on the drum54-. Rotary disc brake elements 62 are attached to the reel 27 in asimilar fashion and the stators 61 and rotors 62 are radiallyinterleaved with each other across brake chamber 56. Hydraulic lines 63attached to the brake cylinders 60 connect with an hydraulic system(FIGURE 9) provided for operating the brakes in accordance with therequirements of the missile arresting application.

Attention is now directed particularly to the hydraulic system foroperating the disc brakes shown schematically in FIGURE 9 and it will beunderstood that while the description is with respect to the systemassociated with arresting engine 25, an identical system would beprovided for arresting engine 26 and the two engines would be operatedin unison. In FEGURE 9 a static oil reservoir 113 is maintainable underpressure in any known manner. Oil is directed through line 117 toshuttle valve 119. Pressure in branch 117A shifts piston 121 to theright thereby connecting branch line 117B to brake actuator feeder line121F which is connected to hydraulic line 63 of the brake 58. Staticbrake pressure tank 113 maintains slight pressure on brake 58 whichallows tape 29 to be tensioned so as to take up any slack between thereel 27 and the tape to tape connector 40. The gear is now in readinessfor the launch of the missile (FIGURE 8). After launch and at the momentof impact (FIG- URE 1), the bridles 34, 35 become taut and alongitudinal stress wave travels down the purchase members 29, 30, 34,35 in a finite time which, in the previous example, is 0.064 second fornylon tape. When this wave reaches the reels they immediately begin torotate due to their low inertial force. Chain driven by each reel 27,28, is a hydraulic pump 126. As pump 126 is operated hydraulic fluid isdrawn from pump fluid reservoir 129 through line 131 connected on thelow pressure side of the pump through pump 126 and then outwardly on thehigh pressure side of the pump through line 133. Line 137 is tapped fromline 133 at junction 135 and is conected to a valve 139 which is openedat some predetermined value to control the return of fluid through theline 141 to the brake fluid tank 129. The valve opening is calculated tosatisfy the performance requirements and geometrical configuration ofeach application as indicated by parameters such as allowabledecelerating loads, sheave span, missile weight, available runout, etc.For example, if missile weight is high in relation to available runout,the valve 139 will be more closed at the beginning of arrestment; orlikewise if the sheave span is wide so as to increase braking torque tocompensate for a lower tape tension due to the geometry of the system.The fluid also is carried in line 143 through check valve 145 to theright side of the spool valve 119. As fluid pressure builds in thesystem corresponding to the increase of velocity of the reel, the staticpressure side of the valve 119 applied by line 117A is overcome and thespool piston 121 is shifted to connect feeder lines 134F and 121Fwhereupon the pressure from the pump 126 replaces the minimal staticpressure from the static brake pressure tank 113. 7

It is important that the reels begin to rotate as quickly as possiblewhen they sense that the system has been impacted by the missile. Thusthe static pressure is very slight and while technically speaking thereels are not free wheeling, the static pressure can be disregarded as aload on the system when compared to the inertial forces resistingrotation by virtue of the reel masses and the complement of purchasetape. After the reels are accelerated to a tape payout speed equal tothe velocity of the missile, the pump 126 will be delivering pressure ata value that exceeds that from tank 113 which signals the approachingend of the dynamic period and the beginning of the programmed brakingperiod. This is the point of transition between the initial erratic andthe flattened portion on the performance curve shown in FIGURE 5. Thearrangement is such that as the reels 25, 2e rotate, the length ofmoment arm due to the tape Wrappings is decreasing. This tends toincrease tape tension because tension is equal to torque divided bymoment arm. Thus, to compensate for this decrease in moment arm, it isnecessary to decrease the torque by reducing brake pressure. This isaccomplished automatically by slower rotation of the reels 25, 26 andpump 126 due to the progressive decrease in missile velocity the netresult is that the tension of the tape during the programmed brakingremains substantially constant. Thus, due to the speed sensingcharacteristic of the hydraulic systems, it is unnecessary tosynchronize the pump drives of reels 25 and 25 for, if reel 25overspeeds for example, it will apply its brakes harder causing it toslow down, while on the other hand, if it underspeeds, then the pressureon the brakes will be relieved permitting the reel to speed up acompensating amount.

It is now necessary to consider the programmed braking cycle in moredetail. The programming means comprises a gear box 149 chain driven fromthe reel to rotate a cam 151 which in turn operates a cam follower 153to open and close the valve 139 which thereby acts as a secondarycontrol On the pressure in the hydraulic system. Due to the inherentspeed sensing characteristics of the system a constant tension ismaintained without having to manipulate the valve 139 any great extentduring the braking cycle. As a result the strain (stretch) placed in thetape during the dynamic period has had no opportunity to be relieved andas the missile approaches a stalling speed near its apogee, this strainwill be unloaded through the missile and guide cables 16, 17 possiblywith sufficient force to collapse the boom 18.

In accordance with the invention, this is prevented by a brakeprogramming especially tailored for missile arresting applications. Cam151 has a profile as shown in 9 FIGURE 10 divided into arcuate sectionsA, B, C. A reciprocal valve element 160 in the valve 139 restricts theflow in line 137 back to the reservoir 129 and during the programmedbraking period corresponding to cam section A is maintained at apredetermined value as previously mentioned with the pump 12.6 varyingthe brake pressure as explained above. As the missile is slowed to astalling speed, the cam profile exhibits a sharp drop at the section Bwinch permits the cam follower 153 to rise under the influence of spring161 so that the valve element 16!) becomes wide open releasing thepressure on the reel brakes. With this type of valve there is a chamber162 behind the valve element 160 which contains hydraulic fluid underpressure from line 137 delivered through longitudinal ports 164 topartially balance the valve element 164 and increase its responsiveness.A plurality of these ports are required so that the valve element 160 isquickly responsive to the change in cam profile at section B, howeverany other means which insures that the cam follower 153 will track onthe cam may be used. The effect of the cam profile drop at section B isto instantly release or dump substantially all of the brake pressure sothat the reel accelerates rapidly to feed additional tape into thesystem during the period covered by cam section C. Thus all the stressis taken out of the purchase members 29, 30, 34, 35 so that as themissile reaches zero velocity, there will be no downward force acting onit. Points on the cam profile are related to exact locations in runoutvia the direct drive from the reel. By this means, it is possible tomaintain pressure for the proper length of time during portion A of thecam profile and to precisely control the pressure dump location betweenA and C. The missile is now slowed to a stalling velocity which meansthat without any additional braking it will come to a stop under theinfluence of gravitational forces alone. As the missile reaches zerovelocity, the cam 151 has rotated beyond section C to the abrupt rise at165 where it closes the valve element 160 causing the pressure from pump126 to rise, thus reapplying the reel brakes so as to prevent tapebacklash which would result by the reel continuing to rotate while themissile had come to a stop. As the reels stop so also does the pump 126whereupon the missile can be lowered by winch and cable mechanism 19 andthe tapes rewound by the reels by means of the retraction engine 38. Inorder to return the system to the initial condition, normally closedmanual shutoff valve 155 (FZGURE 9) is opened to relieve the operatingpressure on the one side of the spool valve 119 and thereby permit thepressure from the static brake pressure tank 113 to again shift thepiston 121 to reestablish static pressure on the reel brakes. Line 143is provided with a relief valve 157 for overpressure protection.

The critical periods in missile arrestment are the dynamic period whilethe reels are being accelerated and at the conclusion of the brakingcycle when the reel brakes are released to dissipate the stresses whichhave built up in the purchase tape returning the system to a neutralcondition. Relative to the dynamic period there are three parameterswhich govern the maximum loads in this area:

(1) Sheave to sheave span which controls, in conjunction with the freeflight distance, the oblique impact angle beta (2) the length ofpurchase tape between missile and arresting engine, and (3) thecross-sectional area of the tape. Depending upon the maximum permissibleload which the missile can withstand, and the imposed nominal strokedistance, the final arrangement is of course flexible between a range ofsheave span and purchase length configurations which may be selected asa matter of practical compromise in order to maintain the arrestingloads below those which the missile can withstand. A narrower sheavespan could be accommodated with a longer purchase length and vice versa;otherwise stated, the narrower the sheave span, the more oblique will bethe angle of engagement of the missile. Consequently a lower angle betaresults in a higher tension which must be accommodated by longerpurchase tape lengths to supply the necessary feed in during the dynamicperiod. Also, any increase in the load carrying capability of themissile would be reflected in a more compact installation. For example,if the limiting factor is vehicle load, then the sheave to sheave spanmust be wider so that the maxi-mum load on the missile is not exceeded,however if the missile can take additional load then the sheave span canbe brought in closer and the limiting factor then becomes thepermissible strain on the purchase tape. As a general rule of practicefor a missile stroke of 100 to 200 feet above ground, the sheave spacingshould be such as to provide an angle of beta of between 20 and 60degrees for nylon tape and preferably around 60 degrees.

Another important factor is the inertia of the reel and its stored tapecomplement during the dynamic period. The tape tension generated duringthis period is a function of accelerating the reel and tape masses. Assoon as the reel is accelerated, the hydraulic system will programpressure to the friction brakes and maintain desired loads until the endof the arrestment. However, during the dynamic period it can be easilyrealized that if the reel inertia is high, then other compensatingfactors must be made, such as spreading out the sheaves resulting in aless compact system. Thus, it may be appreciated that there are a numberof variables which combine to affect the actual engineering of a missilearresting installation and it is not the province of the presentinvention to describe all of the possible situations which could occur,however, by visualizing what may be termed the extreme oppositeconditions, the in between arrangements can be readily accepted.

For example, two missile arresting installations are illustrated inFIGURES 11 and 12. FIGURES 11 and 11A represent what may be called thedegree beta situation in which a missile 13A is launched below groundfor a free flight distance before engaging with a horizontal pendant Pstretched at ground level in the path of the missile 13A. Thearrangement as shown in FIG- URE 11A is such that the missile 13A passesbetween two parallel strands of the pendant and has diametricallyopposed hooks L adapted to engage each strand upon impact. A guide cableand boom arrangement may be provided similar to that shown in FIGURE 1to prevent the missile from falling back to the ground.

FIGURE 12 represents what might be termed the zero beta situation inwhich a missile 13B is launched vertically for a free flight perioddetermined by the length of slack bridle extension S and carries aloftwith it a single purchase tape T paid out vertically directly behind themissile over a sheave 31' from a single arresting engine 25. The tape Tis releasably anchored by the tape to tape connector 4d in a mannersimilar to that described in reference to FIGURE 8.

Having now described the invention and the preferred embodiment thereof,it should be appreciated by those skilled in the arts that obviousvariations are intended to fall within the scope of the claims and becovered thereby except insofar as limited by the prior art.

We claim:

1. An apparatus for arresting a missile while in free flight within aprescribed distance comprising a rotatable reel,

a linear purchase tape considerably wider than it is thick so as topermit coiling upon itself about the reel in ever increasingconvolutions having its running end releasably anchored adjacent thepath of flight of the missile,

means carried by the missile for engaging said running end of thepurchase tape after a period of sustained free flight carrying it aloftwith the missile,

the tape being woven of synthetic yarns having a sufficiently lowmodulus of elasticity so that the portion paid out between the reel andmissile will stretch longitudinally and maintain engagement with the 11missile until the reel can accelerate at the demand rate per secondrequired in order to keep the imposed stress and strain Within safelimits in said tape,

braking means attached to the reel for reducing the rate of tape payoutthus slowing the missile to a stalling speed, and

speed sensing control means arranged to program the retarding force ofthe braking means so that energy stored in the tape as stretch, isrelieved prior to bringing the missile to a controlled stop at itsapogee.

2. An apparatus for arresting a missile as set forth in claim 1including a plurality of rotatable reel and purchase tape units eachpositioned equidistant from the missile launch site and the running endof each purchase tape being releasably anchored adjacent the point oflaunch,

a loose bridle extension the length of which substantially equals saidfree flight distance of the missile connecting the running end of eachpurchase tape thereto, and

a plurality of sheaves positioned equidistant from the missile launchsite through which the payout of each purchase tape occurs as it iscarried aloft by the missile, the running end of each purchase tapebeing lifted from its anchored position when the missile has reached thelimit of free flight imposed on it by said bridle extensions and beingtensioned at an oblique angle to the missile flight path.

3. An apparatus for arresting a missile as set forth in claim 2 whereinthe missile is launched vertically and said oblique angle in between and60 degrees.

4. An apparatus as set forth in claim 1 wherein the purchase tape iswoven of yarn made of polytetrafluorethylene.

5. A missile arresting gear adapted for use in arresting a missile whilein flight comprising a stator member,

a rotatable reel journaled adjacent its opposite ends on the statormember and being radially spaced therefrom to define a brake chambertherebetween,

a plurality of radially extending rotary friction disc elements in thebrake chamber attached to the reel,

a plurality of stationary friction disc elements interleaved with therotary friction disc elements to provide a rotary friction disc brake,

hydraulic acautor means at the opposite ends of said brake chamber forapplying braking pressure upon said friction disc elements so as toretard rotation of said reel,

a purchase tape considerably wider than it is thick so as to permitcoiling upon itself about the reel in ever increasing convolutionshaving its running end releasably anchored adjacent the point of launchof said missile,

a loose bridle extension the length of which substantially equals apredetermined free flight distance of the missile connecting the runningend of the purchase tape onto the missile whereby as the bridleextension becomes taut after launching the missile the purchase tape istensioned until the reel can be accelerated to feed additional tape atthe demand rate per second required in order to keep imposed stressesand strains on the tape within safe limits, and

an hydraulic control system including cam and valve means providing aprogrammed braking pressure after the reel has been accelerated so as toretard payout of said purchase tape slowing the missile to a stallingvelocity and thereafter reducing the braking pressure so as to permitrapid tape feed in relieving the tape stress and strain just prior tothe missile coming to rest at its apogee.

6. A missile arresting gear as set forth in claim 5 wherein said cam andvalve means comprise a rotatable cam lobe driven at a speed proportionalto the speed of said reel,

a cam follower biased against said cam lobe, and

a valve element arranged to increase or decrease braking pressure inaccordance with the position of the cam follower on the cam lobe, saidcam lobe having a ZOne of rapid pressure drop as the missile reaches astalling speed followed by rapid pressure increase just as the missilecomes to rest under the influence of gravitational forces.

7. A method for arresting a missile comprising the steps of restrainingthe missile after a period of free flight with a woven purchase tapemade of synthetic yarn placed in tension by means of oblique impact withthe missile,

paying out additional tape from a reel upon which the tape is coiled ata rate sufficient to prevent excessive tape strain,

reducing the rate of tape payout after the reel has accelerated to'missile velocity and thereby slowing the missile to a stalling speed,

increasing the rate of tape payout as the missile approaches zerovelocity to relieve tape stress, and

halting tape payout after the strain has been relieved and as themissile comes to rest at its apogee.

References Cited by the Examiner UNITED STATES PATENTS 9/1962 Siegel etal. 73-167 3/1963 Shaller 73l67

1. AN APPARATUS FOR ARRESTING A MISSILE WHILE IN FREE FLIGHT WITHIN APRESCRIBED DISTANCE COMPRISING A ROTATABLE REEL, A LINEAR PURCHASE TAPECONSIDERABLY WIDER THAN IT IS THICK SO AS TO PERMIT COILING UPON ITSELFABOUT THE REEL IN EVER INCREASING CONVOLUTIONS HAVING ITS RUNNING ENDRELEASABLY ANCHORED ADJACENT THE PATH OF FLIGHT OF THE MISSILE, MEANSCARRIED BY THE MISSILE FOR ENGAGING SAID RUNNING END OF THE PURCHASETAPE AFTER A PERIOD OF SUSTAINED FREE FLIGHT CARRYING IT ALOFT WITH THEMISSILE, THE TAPE BEING WOVEN OF SYNTHETIC YARNS HAVING A SUFFICIENTLYLOW MODULUS OF ELASTICITY SO THAT THE PORTION PAID OUT BETWEEN THE REELAND MISSILE WILL STRETCH LONGITUDINALLY AND MAINTAIN ENGAGEMENT WITH THEMISSILE UNTIL THE REEL CAN ACCELERATE AT THE DEMAND RATE PER SECONDREQUIRED IN ORDER TO KEEP THE IMPOSED STRESS AND STRAIN WITHIN SAFELIMITS IN SAID TAPE, BRAKING MEANS ATTACHED TO THE REEL FOR REDUCING THERATE OF TAPE PAYOUT THUS SLOWING THE MISSILE TO A STALLING SPEED, ANDSPEED SENSING CONTROL MEANS ARRANGED TO PROGRAM THE RETARDING FORCE OFTHE BRAKING MEANS SO THAT ENERGY STORED IN THE TAPE AS STRETCH, ISRELIEVED PRIOR TO BRINGING THE MISSILE TO A CONTROLLED STOP AT ITSAPOGEE.